This invention relates generally to the cryogenic rectification of air and is particularly useful for the production of ultra high purity clean dry air.
Clean dry air is used as a utility fluid in manufacturing processes. For example, clean dry air is used in the manufacture of semiconductors in such applications as pneumatic valve actuation, for driving small motors and for cleaning equipment parts.
In the cryogenic separation of air to produce nitrogen and certain other pure gases which may be used in semiconductor manufacturing operations, air is first cleaned of high boiling impurities such as carbon dioxide and water vapor prior to being passed into the column or columns of the cryogenic air separation plant. Clean dry air is taken from this cleaned feed to the cryogenic air separation plant for use in the semiconductor manufacturing operation.
Recently several new applications have emerged where clean dry air is used directly in the semiconductor manufacturing equipment. Examples of such new applications include the use of clean dry air as a sweep gas to prevent build up of unwanted contaminants, and the use of clean dry air as a coolant to remove heat from the equipment. Clean dry air for these applications is required at a much higher purity than was previously needed.
Accordingly it is an object of this invention to provide an improved system for producing clean dry air.
It is another object of this invention to provide a system which can effectively produce clean dry air having an ultra high purity.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing ultra high purity clean dry air comprising:
(A) passing feed air at a feed air level into a cryogenic rectification column and producing by cryogenic rectification within the column a nitrogen-rich fluid and an oxygen-enriched fluid;
(B) withdrawing nitrogen-rich fluid from the upper portion of the column, and withdrawing oxygen-enriched fluid from the lower portion of the column;
(C) withdrawing liquid having an oxygen concentration within the range of from 10 to 50 mole percent from the column at a withdrawal level which is within the range of from 1 to 25 equilibrium stages above the feed air level; and
(D) vaporizing the withdrawn liquid and recovering the resulting vapor as ultra high purity clean dry air.
Another aspect of the invention is:
Apparatus for producing ultra high purity clean dry air comprising:
(A) a heat exchanger, a cryogenic rectification column, and means for passing feed air into the cryogenic rectification column at a feed air level;
(B) means for withdrawing fluid from the upper portion of the cryogenic rectification column, and means for withdrawing fluid from the lower portion of the cryogenic rectification column;
(C) means for passing liquid withdrawn from the cryogenic rectification column at a level within the range of from 1 to 25 equilibrium stages above the feed air level to the heat exchanger; and
(D) means for recovering ultra high purity clean dry air from the heat exchanger.
As used herein the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen and nitrogen, such as ambient air.
As used herein the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. 
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the term xe2x80x9ctop condenserxe2x80x9d means a heat exchange device that generates column downflow liquid from column vapor.
As used herein the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein the term xe2x80x9csubcoolingxe2x80x9d means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.
As used herein the term xe2x80x9ctopxe2x80x9d when referring to a column means that section of the column above the column mass transfer internals, i.e. trays or packing.
As used herein the term xe2x80x9cbottomxe2x80x9d when referring to a column means that section of the column below the column mass transfer internals, i.e. trays or packing.
As used herein the term xe2x80x9cultra high purity clean dry airxe2x80x9d means a fluid which comprises from 10 to 50 mole percent oxygen with the balance comprised essentially of nitrogen and containing a total of less than 10,000 parts per billion (ppb), more preferably less than 100 ppb, most preferably less than 10 ppb of hydrogen, carbon monoxide, water, carbon dioxide and hydrocarbon impurities.
As used herein the term xe2x80x9ctrayxe2x80x9d means a contacting stage, which is not necessarily an equilibrium stage, and may mean other contacting apparatus such as packing having a separation capability equivalent to one tray.
As used herein the term xe2x80x9cequilibrium stagexe2x80x9d means a vapor-liquid contacting stage whereby the vapor and liquid leaving the stage are in mass transfer equilibrium, e.g. a tray having 100 percent efficiency or a packing element height equivalent to one theoretical plate (HETP).
As used herein the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the mid point of the column.
As used herein the term xe2x80x9chigh purity nitrogenxe2x80x9d means a fluid having a nitrogen concentration of at least 99 mole percent, preferably at least 99.9 mole percent, most preferably at least 99.999 mole percent. A particularly desirable form of high purity nitrogen is ultra high purity nitrogen which is a fluid having a nitrogen concentration of at least 99.99999 mole percent.